Tottenham in danger of being left behind as top-six rivals all strengthen

The sound of windows closing is causing nervousness around White Hart Lane. The summer transfer period is reaching its end with Tottenham Hotspur struggling to get any business done. Mauricio Pochettino is frustrated.

The bigger concern for the Spurs manager, beyond this summer, is that a window of opportunity may be closing. After three years of progress, this could be the season when Tottenham's momentum stalls.

Pochettino's style relies on relentless energy and the 46-year-old's training methods are notoriously exacting. Spurs sent 12 players to the World Cup, with nine of them representing nations that reached the semifinals. No other club had so many players who went so deep into the tournament.

Harry Kane, in particular, appeared jaded during the knockout rounds and in need of a break but he's not the only one. The Tottenham squad requires an injection of new blood. Unless chairman Daniel Levy pulls off a late transfer coup or two, Pochettino will have to make do with largely the same group of players as last season.

Levy has surprised the football world in the past with last-gasp transactions but there are a number of factors that suggest it will be harder for Spurs this year. The move back to White Hart Lane, and a spectacular new £850 million stadium, has generated great optimism in north London. It has also forced the club to take out a £400m loan. It means that Tottenham need to sell players before they can afford to buy. Everyone in the game knows this, meaning Levy does not have much bargaining power.

The problem is exacerbated by the early closure of the transfer window in England. Foreign sides have until the end of August to get their deals over the line. They can afford to wait. Spurs cannot.

Tottenham are desperate to move out Toby Alderweireld. The centre-back has a clause in his contract that allows him to leave for £25m next summer and his relationship with Pochettino is fractured. Manchester United will pay more than double that fee this week if the deal can be completed, but it may be too late for Spurs to use the cash for reinforcements.

A number of Spurs players have been available all summer but potential buyers are reluctant to pay big money for men Pochettino wants to offload. Danny Rose is another who is out of favour with the manager, while Moussa Sissoko and Victor Wanyama have also been touted around. Mousa Dembele will likely head to China. Any of these players could leave before the end of August but by then, the opportunity to buy will have gone.

Mauricio Pochettino, Tottenham manager, really needs reinforcements if his team is to be successful. FREDERIC J. BROWN/AFP/Getty Images

Pochettino's first XI remains strong but the Argentine needs reliable backups in case of injury or if fatigue begins to show. The seemingly perennial problem of finding a goalscorer who can take some of the weight off Kane remains. There are issues in midfield that the arrival of Jack Grealish from Aston Villa will not solve. Equally, the squad is thin in defence, given the eagerness of Alderweireld and Rose to exit the club.

There is likely to be an adjustment period to the new stadium, too. Tottenham coped admirably during their spell at Wembley and the team and the supporters will appreciate the sense of stability the return to White Hart Lane will bring. However, it may take a month or two for the side to feel comfortable and create an environment that gives a discernible home advantage.

Pochettino has a huge task to maintain the upward trajectory. Manchester City remain the Premier League's best side and Liverpool are likely to improve. Jose Mourinho's United managed to finish above Tottenham last season despite an underwhelming campaign at Old Trafford. Given the events of the summer, it is hard to see Spurs finishing above these three rivals.

There have been massive changes at Arsenal. While Unai Emery backed into the job at the Emirates -- he was originally their third choice -- the new manager will bring a structure and tactical sense that the Gunners lacked in Arsene Wenger's waning years. They will be better. Chelsea, with Maurizio Sarri at the helm, should have more purpose than in the tail end of Antonio Conte's tenure too.

In some small way, each of Tottenham's top-six rivals have made steps forward this summer. Spurs have stood still. A frantic scramble on deadline day is unlikely to change that situation.